1. Field of the Invention
This invention relates to an image-forming apparatus such as an image display apparatus comprising an electron source.
2. Related Background Art
CRTs (cathode ray tubes) are typical image-forming apparatus that utilize electron beams and have been used widely.
In recent years, flat type display apparatus using liquid crystal have been gaining popularity, and gradually replacing CRTs. However, they are not emission type and accompanied by a number of problems including the need of a back light, and hence there has been a strong demand for emission type display apparatus. While plasma displays are commercially available currently as emission type displays, they are based on a principle different from CRTs for light emission and are not comparable in terms of the contrast of the displayed image and the coloring performance of the apparatus. Meanwhile, efforts have been paid for research and development in the field of realizing a flat type image-forming apparatus by arranging a plurality of electron-emitting devices that is comparable with a CRT in terms of the quality of the displayed image. For example, Japanese Patent Application Laid-Open No. 4-163833 discloses a flat type electron beam image-forming apparatus realized by containing linear thermionic cathodes and complex electrode structures in a vacuum envelope.
With an image-forming apparatus comprising an electron source, the electron beams emitted from the electron source to strike an image-forming member can partly collide with the inner wall of the vacuum envelope to make it emit secondary electrons and become charged up to raise the electric potential at the local areas of the inner wall hit by electron beams. Then, the vacuum envelope shows a distorted potential distribution to produce not only unstable electron beam trajectories but also internal electric discharges to degrade and eventually destroy the apparatus.
The charged up areas come to show a raised electric potential and draw electrons, which by turn further raise the potential of the areas until they come to discharge electrons along the inner wall of the vacuum envelope. Known methods of preventing charge-ups and subsequent discharges from taking place on the inner wall of the vacuum envelope include forming an anti-charge film having an appropriate impedance on the inner wall of the vacuum envelope. Japanese Patent Application Laid-Open No. 4-163833 discloses an image-forming apparatus comprising an electroconductive layer of a high impedance electroconductive material arranged on the lateral sides of the inner wall of the glass envelope of the apparatus.
However, a flat type electron beam image-forming apparatus as described in Japanese Patent Application Laid-Open No. 4-163833 has a considerable depth because the glass envelope of the apparatus contains specifically designed structures including horizontal and vertical deflecting electrodes in it. On the other hand, there is a demand for electron beam image-forming apparatus to be used as portable information processing terminals that are as thin and light weight as a liquid crystal display.
In line with the efforts for realizing very thin image-forming apparatus, the applicant of the present patent application has achieved a number of improvements for surface conduction electron-emitting devices and image-forming apparatus comprising such devices. For example, Japanese Patent Application Laid-Open No. 7-235255 describes an electron-emitting device having a simple configuration. Such devices can be arranged over a relatively large area in large numbers to realize a very thin electron beam image-forming apparatus without using complex structures such as electrode structures.
In an image-forming apparatus of the type under consideration, a voltage is applied between the electron source and the image-forming member to accelerate electrons. If ordinary fluorescent bodies are used for the image-forming member, this voltage is desirably raised at least to a level of several kV in order to provide the emitted light with a desired coloring effect.
Then, in a very thin image-forming apparatus, the risk of electric discharge rises high because the inner wall of the vacuum envelope has only a short length between the image-forming member and the electron source.
More specifically, as a voltage is applied between the image-forming member and the electron source to accelerate electrons, a strong electric field is generated along the inner wall of the vacuum envelope particularly when the inner wall of the vacuum envelope has only a short length between the image-forming member and the electron source. As described earlier, the electron beams emitted from the electron source can partly collide with the inner wall of the vacuum envelope to make it emit secondary electrons and become charged up to raise the electric potential at the local areas of the inner wall hit by electron beams. Then, some of the secondary electrons accelerated by the strong electric field can strike the inner wall of the vacuum envelope to give rise to recurrence of the charge-up and the emission of secondary electrons.
Thus, there exists a need for improving image-forming apparatus if they are to be made ever thinner because of the risk of electric discharge.
If such an electric discharge takes place along the inner wall of the vacuum envelope, a large electric current temporarily appears and mainly flows into the electron source and then down to the ground through the wires arranged in the electron source.
Then, if the electric current flows through all or part of the electron-emitting devices of the electron source with an intensity that exceeds the allowable limit for the normal operation of driving the devices, their performance can become degraded and, in some cases, some of the devices can become destroyed. Then, the image displayed on the image-forming apparatus can be lost, if partly, to remarkably degrade the quality of the image and make the image-forming apparatus no longer operational.
Additionally, the electron source drive circuit can also be damaged if the electric current produced by the electric discharge flows into the circuit by way of the wires connected thereto.
In view of the above identified technological problems of known image-forming apparatus of the type under consideration, it is therefore the principal object of the present invention to provide an image-forming apparatus comprising an electron source that can minimize the risk of degradation and damage of the electron source and the electron source drive circuit if electric discharges occur in the apparatus.
According to the invention, there is provided an image-forming apparatus comprising an envelope, an electron source and an image-forming member arranged within the envelope and an electron source drive circuit, an electroconductive member arranged on the inner wall surface of the envelope between the electron source and the image-forming member and an electric current flow path A extending between the electroconductive member and the ground without passing through any of the electron source and the drive circuit and the electric current flow path A has a resistance lower than the resistance of another electric current flow path B extending between the electroconductive member and the ground by way of the electron source or the drive circuit.